1. Field
The present disclosure relates generally to electric motors and to the control of electric motors. More particularly, the present disclosure relates to controlling an electric motor to reduce instability when the motor is providing power to a power source and to protect the power source from undesired current levels.
2. Background
Aircraft may employ various electronic devices and systems to perform various functions on the aircraft. For example, without limitation, electric motors on an aircraft may be used to move flight control surfaces, to raise and lower landing gear, and to perform other functions on the aircraft. Power for the electric motors and other electronic systems and devices on an aircraft may be provided by an aircraft power system.
An example of a type of electric motor used on aircraft, and for many other applications, is a brushless DC electric motor. This type of motor is also known as an electronically commutated motor. A brushless DC motor may comprise, for example, a permanent magnet synchronous motor, a switched reluctance motor, or an induction motor. Brushless DC motors are powered by a direct current (DC) electrical power source via an inverter which is controlled to provide a switching power signal to drive the motor.
For example, a brushless DC motor may include permanent magnets which rotate and a fixed armature which comprises the stator windings. An electronic controller continually switches the phase of power provided to the windings to keep the motor turning. The controller may employ a solid state circuit to provide timed power distribution to the motor windings.
To direct the rotor rotation, the controller for a brushless DC motor requires some means of determining the rotor's orientation relative to the stator windings. Some brushless DC motors use Hall effect sensors or a rotary encoder to directly measure the position of the rotor. Others measure the back electromotive force (EMF) in the undriven windings to infer the rotor position. Controllers of this latter type are often called sensorless controllers. Other sensorless controllers are capable of measuring winding saturation caused by the position of the magnets to infer the rotor position.
The controller for a brushless DC motor may provide bi-directional outputs to control the driving of DC power to the motor windings. The outputs may be controlled by a logic circuit. Simple controllers may employ comparators to determine when the output phase should be advanced. More advanced controllers may employ a microcontroller to manage acceleration, control speed, and fine tune-motor efficiency. Motor controllers of this type may be referred to as electronic speed controllers.
A controller may control the power that is provided to the windings of a DC motor by controlling the switches in a switch bridge. The switch bridge couples the DC power source to the windings of the DC motor. For example, a three-phase switch bridge may have six switches arranged to form three parallel half H-bridges for coupling the DC power source to three motor windings of a DC motor. The switches of the switch bridge may be controlled by the controller to drive a current in either direction on each of the motor windings. For example, without limitation, the switch bridge may be implemented using solid state switching devices such as metal-oxide-semiconductor field-effect transistors (MOSFETs).
Various methods may be used to control the switches in a switch bridge to modulate the current in the windings of a brushless DC motor. However, existing methods for modulating the current in the motor windings of a brushless DC motor may have various drawbacks and limitations. A method for controlling the current in the windings of a brushless DC motor that overcomes these drawbacks and limitations is desirable.
The rotor in a brushless DC motor may be controlled to rotate in either direction, clockwise or counter-clockwise. The current in the windings of the motor may be controlled to produce torque on the rotor in either the clockwise or counter-clockwise direction. The current in the windings may be controlled to produce torque on the rotor that is either in the same direction as the direction of rotation of the rotor or in the opposite direction from the direction of rotation of the rotor at a particular point in time.
Operation of a brushless DC motor thus may be described with reference to four quadrants. For example, in a first quadrant of operation, the rotor may be rotating in a clockwise direction and the current in the motor windings may produce torque on the rotor in the same clockwise direction. In a second quadrant of operation, the rotor may be rotating in a counter-clockwise direction and the current in the motor windings may produce torque on the rotor in the opposite clockwise direction. In a third quadrant of operation, the rotor may be rotating in a counter-clockwise direction and the current in the motor windings may produce torque on the rotor in the same counter-clockwise direction. In the fourth quadrant of operation, the rotor may be rotating in a clockwise direction and the current in the motor windings may produce torque on the rotor in the opposite counter-clockwise direction.
When a motor is operating in the second and fourth quadrants of operation, the motor may provide power back to the power source. In this case, the motor may be said to be regenerating or in a regenerating mode of operation. It is desirable to reduce or prevent undesired conditions that may occur when the motor is regenerating. For example, it is desirable to reduce or prevent instability that may occur when the motor is regenerating. It is also desirable to reduce or prevent undesired current flow between the motor and the power source when the motor is operating in various quadrants, including when the motor is regenerating. Accordingly, it would be desirable to have a method and apparatus that take into account one or more of the issues discussed above as well as possibly other issues.